Suction-air systems of the basic type described above are known. Thus, for example, German Patent Publication DE 24 26 961 B2 describes a fire detector which is arranged in a suction-air system of a rotor spinning machine. The task of such a fire detector is to monitor fiber fly of the rotor spinning machine transported within a suction-air conduit for any spark formation. Such a spark formation can arise, e.g., because a rotor of the rotor spinning machine overheats, which can cause an ignition of fibers. Such ignited fibers can be transported via the suction-air conduit over relatively large distances, e.g. into air-conditioning equipment, so that considerable fire damage can occur.
It is known that such fire detectors (referred to herein as spark sensors) can be provided with an optical receiver for electromagnetic waves which receiver senses, in particular, infrared signals emitted by a spark. Such electromagnetic waves can be converted into an electrical signal by a photoelement, e.g., a photodiode. This electrical signal can be evaluated with a driver and evaluation circuit. Normally, the photoelements are loaded with a resting current, or zero-signal current, which changes into a switching current after the incidence of an electromagnetic wave to initiate an event-dependent action. This action can be, e.g., an alarm signal to operating personnel of the rotor spinning machine, an automatic deactuation of the rotor spinning machine, the activation of an extinguishing device or the like.
While they are being used according to regulation, spark sensors are exposed to significant stress, so that a checking of the operation of the spark sensors is necessary for reliable operation. As a result of the defined switching states of the spark sensors between the resting current, on the one hand, and the switching current, on the other hand, an electrical check can be performed by means of a testing algorithm via the driver and evaluation circuit. In the case of any short circuit of the leads to the measuring device, a current flows which is greater than the resting current, whereas in the case of an interruption of the lead of the measuring device no current flows. In the case of both error states, an alarm signal can be generated which indicates a failure of the spark sensor.
However, it is a disadvantage that checking of the optical structural elements of the spark sensor is not possible, so that an operational safety of 100% can not be assured by the electrical test.